Mitochondrial function requires the coordinate synthesis of proteins encoded in the nucleus and the organellar genome. The long-term goal of this research is to understand how mitochondrial gene expression is regulated. The focus is on the subset of yeast nuclear genes encoding proteins that enter the organelle to participate in post-transcriptional processes. Each of the seven mitochondrial mRNAs require several message-specific factors that regulate intron removal, mRNA stability and translation. Cbpl has been studied in-depth. It is required specifically for cytochrome b mRNA stability. Recently Cbpl was found to be part of a large multisubunit complex. Cbs2, a factor that specifically promotes translation of COB, is also in a large complex of similar mass. Specific Aim 1 is to analyze the composition and function of these large complexes in RNA protection and translation. Specific Aim 2 is to determine if the two functions of Cbpl, message stabilization and translation, are separable. Specific Aim 3 is to characterize Aep3 in its roles in protection and translation of ATP8/6 mRNA. In addition to the studies on message-specific factors that protect mRNAs and promote their translation, several general factors are important in the turnover of the mRNAs. Overexpression of the Cbt1 protein suppresses mRNA instability, whereas deletion of the CBT1 gene results in defective processing of precursor RNAs. Three other mitochondrial proteins have been implicated in these processing events and mRNA turnover. Cbt1 is in a large complex of proteins of 500,000 daltons. Specific Aim 4 is to analyze the composition and function of this large complex in precursor RNA processing and mRNA turnover. This work will further our general understanding of RNA expression in mitochondria. Many human diseases are caused by mutations in nuclear genes that regulate mitochondrial gene expression. Yeast is a good model system to understand processes that may be affected in human mitochondrial disease syndromes.